A hotfix will be applied to adjust how Vengeance stacks for tanks of all classes:

Vengeance should now ramp up more quickly. Avoidance will now grant Vengeance based on the average damage of the avoided NPC auto-attack, instead of just refreshing existing Vengeance. This does not apply to enemy special attacks.

This will be noted in the next hotfix update, but we wanted to let you know immediately, as this change will be applied sometime today.